Network Working Group W. Eddy
Internet-Draft MTI Systems
Intended status: Informational E. Davies
Expires: July 30, 2011 Folly Consulting
January 26, 2011
Using Self-Delimiting Numeric Values in Protocolsdraft-irtf-dtnrg-sdnv-08
Abstract
Self-Delimiting Numeric Values (SDNVs) have recently been introduced
as a field type in proposed Delay-Tolerant Networking protocols.
SDNVs encode an arbitrary-length non-negative integer or arbitrary-
length bit-string with minimum overhead. They are intended to
provide protocol flexibility without sacrificing economy, and to
assist in future-proofing protocols under development. This document
describes formats and algorithms for SDNV encoding and decoding,
along with notes on implementation and usage. This document is a
product of the Delay Tolerant Networking Research Group and has been
reviewed by that group. No objections to its publication as an RFC
were raised.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 30, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Internet-Draft Using SDNVs January 20111. Introduction
This document is a product of the Internet Research Task Force (IRTF)
Delay-Tolerant Networking (DTN) Research Group (DTNRG). The document
has received review and support within the DTNRG, as discussed in the
Acknowledgements section of this document.
This document begins by describing the drawbacks of using fixed-width
protocol fields. It then provides some background on the Self-
Delimiting Numeric Values (SDNVs) proposed for use in DTN protocols,
and motivates their potential applicability in other networking
protocols. One example of SDNVs being used outside of the DTN
protocols is in Hixie's Web Socket protocol
[I-D.hixie-thewebsocketprotocol], which includes a binary frame
length indicator field identical to an SDNV. The DTNRG has created
SDNVs to meet the challenges it attempts to solve, and it has been
noted that SDNVs closely resemble certain constructs within ASN.1 and
even older ITU protocols, so the problems are not new or unique to
DTN. SDNVs focus strictly on numeric values or bitstrings, while
other mechanisms have been developed for encoding more complex data
structures, such as ASN.1 encoding rules, and Haverty's MSDTP
[RFC0713]. Because of this focus, SDNVs are can be quickly
implemented with only a small amount of code.
SDNVs are tersely defined in both the bundle protocol [RFC5050] and
LTP [RFC5326] specifications, due to the flow of document production
in the DTNRG. This document clarifies and further explains the
motivations and engineering decisions behind SDNVs.
1.1. Problems with Fixed Value Fields
Protocol designers commonly face an optimization problem in
determining the proper size for header fields. There is a strong
desire to keep fields as small as possible, in order to reduce the
protocol's overhead, and also allow for fast processing. Since
protocols can be used many years (even decades) after they are
designed, and networking technology has tended to change rapidly, it
is not uncommon for the use, deployment, or performance of a
particular protocol to be limited or infringed upon by the length of
some header field being too short. Two well-known examples of this
phenomenon are the TCP advertised receive window, and the IPv4
address length.
TCP segments contain an advertised receive window field that is fixed
at 16 bits [RFC0793], encoding a maximum value of around 65
kilobytes. The purpose of this value is to provide flow control, by
allowing a receiver to specify how many sent bytes its peer can have
outstanding (unacknowledged) at any time, thus allowing the receiver
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to limit its buffer size. As network speeds have grown by several
orders of magnitude since TCP's inception, the combination of the 65
kilobyte maximum advertised window and long round-trip times
prevented TCP senders from being able to achieve the high throughput
that the underlying network supported. This limitation was remedied
through the use of the Window Scale option [RFC1323], which provides
a multiplier for the advertised window field. However, the Window
Scale multiplier is fixed for the duration of the connection,
requires support from each end of a TCP connection, and limits the
precision of the advertised receive window, so this is certainly a
less-than-ideal solution. Because of the field width limit in the
original design however, the Window Scale is necessary for TCP to
reach high sending rates.
An IPv4 address is fixed at 32 bits [RFC0791] (as a historical note,
an early version of the IP header format specification in [IEN21]
used variable-length addresses in multiples of 8-bits up to 120-
bits). Due to the way that subnetting and assignment of address
blocks was performed, the number of IPv4 addresses has been seen as a
limit to the growth of the Internet [Hain05]. Two divergent paths to
solve this problem have been the use of Network Address Translators
(NATs) and the development of IPv6. NATs have caused a number of
other issues and problems [RFC2993], leading to increased complexity
and fragility, as well as forcing workarounds to be engineered for
many other protocols to function within a NATed environment. The
IPv6 solution's transitional work has been underway for several
years, but has still only just begun to have visible impact on the
global Internet.
Of course, in both the case of the TCP receive window and IPv4
address length, the field size chosen by the designers seemed like a
good idea at the time. The fields were more than big enough for the
originally perceived usage of the protocols, and yet were small
enough to allow the headers to remain compact and relatively easy and
efficient to parse on machines of the time. The fixed sizes that
were defined represented a trade off between the scalability of the
protocol versus the overhead and efficiency of processing. In both
cases, these engineering decisions turned out to be painfully
restrictive in the longer term.
1.2. SDNVs for DTN Protocols
In specifications for the DTN Bundle Protocol (BP) [RFC5050] and
Licklider Transmission Protocol (LTP) [RFC5326], SDNVs have been used
for several fields including identifiers, payload/header lengths, and
serial (sequence) numbers. SDNVs were developed for use in these
types of fields, to avoid sending more bytes than needed, as well as
avoiding fixed sizes that may not end up being appropriate. For
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example, since LTP is intended primarily for use in long-delay
interplanetary communications [RFC5325], where links may be fairly
low in capacity, it is desirable to avoid the header overhead of
routinely sending a 64-bit field where a 16-bit field would suffice.
Since many of the nodes implementing LTP are expected to be beyond
the current range of human spaceflight, upgrading their on-board LTP
implementations to use longer values if the defined fields are found
to be too short would also be problematic. Furthermore, extensions
similar in mechanism to TCP's Window Scale option are unsuitable for
use in DTN protocols since, due to high delays, DTN protocols must
avoid handshaking and configuration parameter negotiation to the
greatest extent possible. All of these reasons make the choice of
SDNVs for use in DTN protocols attractive.
1.3. SDNV Usage
In short, an SDNV is simply a way of representing non-negative
integers (both positive integers of arbitrary magnitude and 0),
without expending much unnecessary space. This definition allows
SDNVs to represent many common protocol header fields, such as:
o Random identification fields as used in the IPsec Security
Parameters Index or in IP headers for fragment reassembly (Note:
the 16-bit IP ID field for fragment reassembly was recently found
to be too short in some environments [RFC4963]),
o Sequence numbers as in TCP or SCTP,
o Values used in cryptographic algorithms such as RSA keys, Diffie-
Hellman key-agreement, or coordinates of points on elliptic
curves.
o Message lengths as used in file transfer protocols.
o Nonces and cookies.
As any bit-field can be interpreted as an unsigned integer, SDNVs can
also encode arbitrary-length bit-fields, including bit-fields
representing signed integers or other data types; however, this
document assumes SDNV encoding and decoding in terms of unsigned
integers. Implementations may differ in the interface that they
provide to SDNV encoding and decoding functions, in terms of whether
the values are numeric, bit-fields, etc.; this detail does not alter
the representation or algorithms described in this document.
The use of SDNVs rather than fixed length fields gives protocol
designers the ability to ameliorate the consequences of making
difficult-to-reverse field-sizing decisions, as the SDNV format grows
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and shrinks depending on the particular value encoded. SDNVs do not
necessarily provide optimal encodings for values of any particular
length, however they allow protocol designers to avoid potential
blunders in assigning fixed lengths, and remove the complexity
involved with either negotiating field lengths or constructing
protocol extensions. However, if SDNVs are used to encode bit-
fields, it is essential that the sender and receiver have a
consistent interpretation of the decoded value. This is discussed
further in Section 2.
To our knowledge, at this time, no IETF transport or network-layer
protocol designed for use outside of the DTN domain has proposed to
use SDNVs; however there is no inherent reason not to use SDNVs more
broadly in the future. The two examples cited here, of fields that
have proven too small in general Internet protocols, are only a small
sampling of the much larger set of similar instances that the authors
can think of. Outside the Internet protocols, within ASN.1 and
previous ITU protocols, constructs very similar to SDNVs have been
used for many years due to engineering concerns very similar to those
facing the DTNRG.
Many protocols use a Type-Length-Value method for encoding variable
length fields (e.g. TCP's options format, or many of the fields in
IKEv2). An SDNV is equivalent to combining the length and value
portions of this type of field, with the overhead of the length
portion amortized out over the bytes of the value. The penalty paid
for this in an SDNV may be several extra bytes for long values (e.g.
1024 bit RSA keys). See Section 4 for further discussion and a
comparison.
As is shown in later sections, for large values, the current SDNV
scheme is fairly inefficient in terms of space (1/8 of the bits are
overhead) and not particularly easy to encode/decode in comparison to
alternatives. The best use of SDNVs may often be to define the
Length field of a TLV structure to be an SDNV whose value is the
length of the TLV's Value field. In this way, one can avoid forcing
large numbers from being directly encoded as an SDNV, yet retain the
extensibility that using SDNVs grants.
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Internet-Draft Using SDNVs January 20112. Definition of SDNVs
Early in the work of the DTNRG, it was agreed that the properties of
an SDNV were useful for DTN protocols. The exact SDNV format used by
the DTNRG evolved somewhat over time before the publication of the
initial RFCs on LTP and the BP. An earlier version bore resemblance
to the ASN.1 [ASN1] Basic Encoding Rules (BER) [ASN1-BER] for lengths
(Section 8.1.3 of X.690). The current SDNV format is the one used by
ASN.1 BER for encoding tag identifiers greater than or equal to 31
(Section 8.1.2.4.2 of X.690). A comparison between the current SDNV
format and the early SDNV format is made in Section 4.
The format currently used is very simple. Before encoding, an
integer is represented as a left-to-right bitstring beginning with
its most significant bit, and ending with its least significant bit.
If the bitstring's length is not a multiple of 7, then the string is
left-padded with zeros. When transmitted, the bits are encoded into
a series of bytes. The low-order 7 bits of each byte in the encoded
format are taken left-to-right from the integer's bitstring
representation. The most significant bit of each byte specifies
whether it is the final byte of the encoded value (when it holds a
0), or not (when it holds a 1).
For example:
o 1 (decimal) is represented by the bitstring "0000001" and encoded
as the single byte 0x01 (in hexadecimal)
o 128 is represented by the bitstring "10000001 00000000" and
encoded as the bytes 0x81 followed by 0x00.
o Other values can be found in the test vectors of the source code
in Appendix A
To be perfectly clear, and avoid potential interoperability issues
(as have occurred with ASN.1 BER time values), we explicitly state
two considerations regarding zero-padding. (1) When encoding SDNVs,
any leading (most significant) zero bits in the input number might be
discarded by the SDNV encoder. Protocols that use SDNVs should not
rely on leading-zeros being retained after encoding and decoding
operations. (2) When decoding SDNVs, the relevant number of leading
zeros required to pad up to a machine word or other natural data unit
might be added. These are put in the most-significant positions in
order to not change the value of the number. Protocols using SDNVs
should consider situations where lost zero-padding may be
problematic.
The issues of zero-padding are particularly relevant where an SDNV is
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being used to represent a bit-field to be transmitted by a protocol.
The specification of the protocol and any associated IANA registry
should specify the allocation and usage of bit positions within the
unencoded field. Unassigned and reserved bits in the unencoded field
will be treated as zeros by the SDNV encoding prior to transmission.
Assuming the bit positions are numbered starting from 0 at the least
significant bit position in the integer representation, then if
higher numbered positions in the field contain all zeros, the
encoding process may not transmit these bits explicitly (e.g., if all
the bit positions numbered 7 or higher are zeros then the transmitted
SDNV can consist of just one octet). On reception the decoding
process will treat any untransmitted higher numbered bits as zeros.
To ensure correct operation of the protocol, the sender and receiver
must have a consistent interpretation of the width of the bit-field.
This can be achieved in various ways:
o the bit-field width is implicitly defined by the version of the
protocol in use in the sender and receiver,
o sending the width of the bit-field explicitly in a separate item,
o the higher numbered bits can be safely ignored by the receiver
(e.g., because they represent optimizations), or
o marking the highest numbered bit by prepending a 1 bit to the bit-
field.
The protocol specification must record how the consistent
interpretation is achieved.
The SDNV encoding technique is also known as Variable Byte Encoding
(see Section 5.3.1 of [Manning09]) and is equivalent to Base-128
Elias Gamma Encoding (see Section 5.3.2 of [Manning09] and Section3.5 of [Sayood02]). However the primary motivation for SDNVs is to
provide an extensible protocol framework rather than optimal data
compression which is the motivation behind the other uses of the
technique. [Manning09] points out that the key feature of this
encoding is that it is 'prefix free' meaning that no code is a prefix
of any other, which an alternative way of expressing the self-
delimiting property
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Internet-Draft Using SDNVs January 20113. Basic Algorithms
This section describes some simple algorithms for creating and
parsing SDNV fields. These may not be the most efficient algorithms
possible, however, they are easy to read, understand, and implement.
Appendix A contains Python source code implementing the routines
described here. The algorithms presented here are convenient for
converting between an internal data block and serialized data stream
associated with a transmission device. Other approaches are possible
with different efficiencies and trade-offs.
3.1. Encoding Algorithm
There is a very simple algorithm for the encoding operation that
converts a non-negative integer (value n, of length 1+floor(log n)
bits) into an SDNV. This algorithm takes n as its only argument and
returns a string of bytes:
o (Initial Step) Set a variable X to a byte sharing the least
significant 7 bits of n, and with 0 in the most significant bit,
and a variable Y to n, right-shifted by 7 bits.
o (Recursion Step) If Y == 0, return X. Otherwise, set Z to the
bitwise-or of 0x80 with the 7 least significant bits of Y, and
append Z to X. Right-shift Y by 7 bits and repeat the Recursion
Step.
This encoding algorithm has time complexity of O(log n), since it
takes a number of steps equal to ceil(n/7), and no additional space
beyond the size of the result (8/7 log n) is required. One aspect of
this algorithm is that it assumes strings can be efficiently appended
to new bytes. One way to implement this is to allocate a buffer for
the expected length of the result and fill that buffer one byte at a
time from the right end.
If, for some reason, an implementation requires an encoded SDNV to be
some specific length (possibly related to a machine word), any
leftmost zero-padding included needs to properly set the high-order
bit in each byte of padding.
3.2. Decoding Algorithm
Decoding SDNVs is a more difficult operation than encoding them, due
to the fact that no bound on the resulting value is known until the
SDNV is parsed, at which point the value itself is already known.
This means that if space is allocated for decoding the value of an
SDNV into, it is never known whether this space will be overflowed
until it is 7 bits away from happening.
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(Initial Step) Set the result to 0. Set an index to the first byte
of the encoded SDNV.
(Recursion Step) Shift the result left 7 bits. Add the low-order 7
bits of the value at the index to the result. If the high-order bit
under the pointer is a 1, advance the index by one byte within the
encoded SDNV and repeat the Recursion Step, otherwise return the
current value of the result.
This decoding algorithm takes no more additional space than what is
required for the result (7/8 the length of the SDNV) and the pointer.
The complication is that before the result can be left-shifted in the
Recursion Step, an implementation needs to first make sure that this
won't cause any bits to be lost, and re-allocate a larger piece of
memory for the result, if required. The pure time complexity is the
same as for the encoding algorithm given, but if re-allocation is
needed due to the inability to predict the size of the result,
decoding may be slower.
These decoding steps include removal of any leftmost zero-padding
that might be used by an encoder to create encodings of a certain
length.
3.3. Limitations of Implementations
Because of efficiency considerations or convenience of internal
representation of decoded integers, implementations may choose to
limit the number of bits in SDNVs that they will handle. To avoid
interoperability problems any protocol that uses SDNVs must specify
the largest number of bits in an SDNV that an implementation of that
protocol is expected to handle.
For example Section 4.1 of [RFC5050] specifies that implementations
of the DTN Bundle Protocol are not required to handle SDNVs with more
than 64 bits in their unencoded value. Accordingly integer values
transmitted in SDNVs have an upper limit and SDNV encoded flag fields
must be limited to 64 bit positions in any future revisions of the
protocol unless the restriction is altered.
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Internet-Draft Using SDNVs January 20114. Comparison to Alternatives
This section compares three alternative ways of implementing the
concept of SDNVs: (1) the TLV scheme commonly used in the Internet
family, and many other families of protocols, (2) the old style of
SDNVs (both the SDNV-8 and SDNV-16) defined in an early stage of
LTP's development [BRF04], and (3) the current SDNV format.
The TLV method uses two fixed-length fields to hold the Type and
Length elements that then imply the syntax and semantics of the Value
element. This is only similar to an SDNV in that the value element
can grow or shrink within the bounds capable of being conveyed by the
Length field. Two fundamental differences between TLVs and SDNVs are
that through the Type element, TLVs also contain some notion of what
their contents are semantically, while SDNVs are simply generic non-
negative integers, and protocol engineers still have to choose fixed
field lengths for the Type and Length fields in the TLV format.
Some protocols use TLVs where the value conveyed within the Length
field needs to be decoded into the actual length of the Value field.
This may be accomplished through simple multiplication, left-
shifting, or a look-up table. In any case, this tactic limits the
granularity of the possible Value lengths, and can contribute some
degree of bloat if Values do not fit neatly within the available
decoded Lengths.
In the SDNV format originally used by LTP, parsing the first byte of
the SDNV told an implementation how much space was required to hold
the contained value. There were two different types of SDNVs defined
for different ranges of use. The SDNV-8 type could hold values up to
127 in a single byte, while the SDNV-16 type could hold values up to
32,767 in 2 bytes. Both formats could encode values requiring up to
N bytes in N+2 bytes, where N<127. The major difference between this
old SDNV format and the current SDNV format is that the new format is
not as easily decoded as the old format was, but the new format also
has absolutely no limitation on its length.
The advantage in ease of parsing the old format manifests itself in
two aspects: (1) the size of the value is determinable ahead of time,
in a way equivalent to parsing a TLV, and (2) the actual value is
directly encoded and decoded, without shifting and masking bits as is
required in the new format. For these reasons, the old format
requires less computational overhead to deal with, but is also very
limited, in that it can only hold a 1024-bit number, at maximum.
Since according to IETF Best Current Practices, an asymmetric
cryptography key needed to last for a long term requires using moduli
of over 1228 bits [RFC3766], this could be seen as a severe
limitation of the old-style of SDNVs, which the currently-used style
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does not suffer from.
Table 1 compares the maximum values that can be encoded into SDNVs of
various lengths using the old SDNV-8/16 method and the current SDNV
method. The only place in this table where SDNV-16 is used rather
than SDNV-8 is in the 2-byte row. Starting with a single byte, the
two methods are equivalent, but when using 2 bytes, the old method is
a more compact encoding by one bit. From 3 to 7 bytes of length
though, the current SDNV format is more compact, since it only
requires one bit per byte of overhead, whereas the old format used a
full byte. Thus, at 8 bytes, both schemes are equivalent in
efficiency since they both use 8 bits of overhead. Up to 129 bytes,
the old format is more compact than the current one, although after
this limit it becomes unusable.
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Another aspect of comparison between SDNVs and alternatives using
fixed-length fields is the result of errors in transmission. Bit-
errors in an SDNV can result in either errors in the decoded value,
or parsing errors in subsequent fields of the protocol. In fixed-
length fields, bit errors always result in errors to the decoded
value rather than parsing errors in subsequent fields. If the
decoded values from either type of field encoding (SDNV or fixed-
length) are used as indexes, offsets, or lengths of further fields in
the protocol, similar failures result.
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Internet-Draft Using SDNVs January 20115. Security Considerations
The only security considerations with regards to SDNVs are that code
which parses SDNVs should have bounds-checking logic and be capable
of handling cases where an SDNV's value is beyond the code's ability
to parse. These precautions can prevent potential exploits involving
SDNV decoding routines.
Stephen Farrell noted that very early definitions of SDNVs also
allowed negative integers. This was considered a potential security
hole, since it could expose implementations to underflow attacks
during SDNV decoding. There is a precedent in that many existing TLV
decoders map the Length field to a signed integer and are vulnerable
in this way. An SDNV decoder should be based on unsigned types and
not have this issue.
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Internet-Draft Using SDNVs January 20117. Acknowledgements
Scott Burleigh, Manikantan Ramadas, Michael Demmer, Stephen Farrell,
and other members of the IRTF DTN Research Group contributed to the
development and usage of SDNVs in DTN protocols. George Jones and
Keith Scott from Mitre, Lloyd Wood, Gerardo Izquierdo, Joel Halpern,
Peter TB Brett, Kevin Fall, and Elwyn Davies also contributed useful
comments on and criticisms of this document. DTNRG last call
comments on the draft were sent to the mailing list by Lloyd Wood,
Will Ivancic, Jim Wyllie, William Edwards, Hans Kruse, Janico
Greifenberg, Teemu Karkkainen, Stephen Farrell, and Scott Burleigh.
Further constructive comments were incorporated from Dave Crocker,
Lachlan Andrew and Michael Welzl.
Work on this document was performed at NASA's Glenn Research Center,
in support of the NASA Space Communications Architecture Working
Group (SCAWG), NASA's Earth Science Technology Office (ESTO), and the
FAA/Eurocontrol Future Communications Study (FCS) in the 2005-2007
time frame, while the editor was an employee of Verizon Federal
Network Systems.
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Internet-Draft Using SDNVs January 2011Appendix A. SNDV Python Source Code
# sdnv_decode() takes a string argument (s), which is assumed to be
# an SDNV, and optionally a number (slen) for the maximum number of
# bytes to parse from the string. The function returns a pair of
# the non-negative integer n that is the numeric value encoded in
# the SDNV, and integer that is the distance parsed into the input
# string. If the slen argument is not given (or is not a non-zero
# number) then, s is parsed up to the first byte whose high-order
# bit is 0 -- the length of the SDNV portion of s does not have to
# be pre-computed by calling code. If the slen argument is given
# as a non-zero value, then slen bytes of s are parsed. The value
# for n of -1 is returned for any type of parsing error.
#
# NOTE: In python, integers can be of arbitrary size. In other
# languages, such as C, SDNV-parsing routines should take
# precautions to avoid overflow (e.g. by using the Gnu MP library,
# or similar).
#
def sdnv_decode(s, slen=0):
n = long(0)
for i in range(0, len(s)):
v = ord(s[i])
n = n<<7
n = n + (v & 0x7F)
if v>>7 == 0:
slen = i+1
break
elif i == len(s)-1 or (slen != 0 and i > slen):
n = -1 # reached end of input without seeing end of SDNV
return (n, slen)
# sdnv_encode() returns the SDNV-encoded string that represents n.
# An empty string is returned if n is not a non-negative integer
def sdnv_encode(n):
r = ""
# validate input
if n >= 0 and (type(n) in [type(int(1)), type(long(1))]):
flag = 0
done = False
while not done:
# encode lowest 7 bits from n
newbits = n & 0x7F
n = n>>7
r = chr(newbits + flag) + r
if flag == 0:
flag = 0x80
if n == 0:
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